Embodiments herein generally relate to structures and methods that control the speed of electric motors such as stepper motors, and more particularly to an improved motor and method that utilizes two modes to reduce vibrations and noise of such motors.
Where perfect speed control is required, hybrid stepper motors are often chosen for their precision, in order to minimize vibration. However, even with micro-stepping techniques, vibration continues to be a challenge with popular drive circuitry, as does limitations on the useable range of speed. This disclosure explains how standard drive circuit topology can be used along with enhanced control logic to accomplish very low vibration operation over an unprecedented range of speed.
When a stepper motor is used to produce continuous motion, it periodically updates the electrical signal supplied to the coils. To keep the torque constant and the angular increment regular, the coils of a 2-phase bipolar wound motor (a very common stepper motor arrangement) are driven such that the currents in those coils approximate a sine wave and a cosine wave.
These waveforms, illustrated in FIG. 1, are normally produced using a digital lookup table that is indexed by a counter, and whose output is converted to an analog signal that controls the driver circuit. The same table may be used to produce the signals for both motor coils, when the motor coils are indexed by a pair of pointers (or counters) that are always pointing to positions in the table that are separated by a quarter of the range, hence producing sine and cosine functions of the counter value. As the counter increments at a given frequency, the motor spins at a corresponding speed. If the counter is decrementing, the motor direction is reversed.
With respect to the effects of vibration, free (unloaded) stepper motors can be set to move to any position at any torque by controlling the coil currents. This is sometimes called the electrical position. However the mechanical position of a torque loaded motor will stray from this by a positional error called the load angle, as shown in FIG. 2. The load angle sets the stage for harmonic motion. Since any imperfection causing noise at the resonant frequency can build up this oscillation amplitude, what is needed is a damping mechanism.
One exemplary apparatus embodiment herein that provides damping to reduce such oscillation comprises a dual-mode controller operatively connected to an electric motor. The controller is adapted to supply a pattern of voltage to the electric motor. Further, the controller has an input/output for receiving a desired rotational speed and step frequency of the electric motor. A current feedback loop is connected to the electric motor and the controller.
The controller operates in two different modes, a high-speed, closed-loop mode and a low-speed, open-loop mode. Thus, the controller is adapted to perform closed-loop mode control of coil current of the electric motor by varying the average voltage supplied to the electric motor according to observed feedback current from the current feedback loop, if the desired rotational speed is high enough (e.g., above a predetermined limit). However, when the desired rotational speed is slow enough (e.g., not above the predetermined limit) the controller is adapted to perform open-loop mode control of coil current of the electric motor by setting the average voltage supplied to the electric motor according to values computed from the step frequency, irrespective of the observed feedback current from the current feedback loop. The predetermined limit can be either a step frequency or a revolution frequency.
More specifically, the controller is adapted to perform the closed-loop mode control using feedback-based pulse width modulation (PWM) to control the pattern of voltage supplied to the electric motor so as to maintain sinusoidal current waveforms within the electric motor.
However, the controller performs the open-loop mode control to supply, from the controller to the electric motor, sinusoidal voltage waveforms matching desired sinusoidal current waveforms corresponding to the desired rotational speed. The current feedback loop is adapted to provide over-current protection during the open-loop mode control.
A method embodiment receives the desired rotational speed and step frequency for the electric stepper motor. If the desired rotational speed is above a predetermined limit, the method performs closed-loop mode control of coil current of the electric stepper motor by varying the average voltage supplied to the electric stepper motor according to observed feedback current from the current feedback loop connected to the electric stepper motor. To the contrary, if the desired rotational speed is not above the predetermined limit, the method performs open-loop mode control of coil current of the electric stepper motor by setting the average voltage supplied to the electric stepper motor according to values computed from the step frequency, irrespective of the observed feedback current from the current feedback loop.
These and other features are described in, or are apparent from, the following detailed description.